1. Field of the Invention
The present invention concerns a metallic gasket disposed between the joining faces of a cylinder head and a cylinder block that constitute an internal combustion engine to prevent combustion gas, cooling water, lubricant or the like from leaking.
2. Description of the Prior Art
In an internal combustion engine, a gasket is interposed between a cylinder block and a cylinder head to provide a seal therebetween. The materials of the gasket for a gasoline engine use a mixture of asbestos and rubber and have a spiked steel sheet embedded therein. A bore site peripheral edge covered with a stainless steel sheet and a peripheral edge of bolt holes covered with a copper sheet, respectively, are used as a countermeasure for a gap between the seal and the joining faces of the cylinder block and the cylinder head.
Further, for a diesel engine, a material comprising asbestos, rubber and a wire gauge embedded therein is used as a countermeasure for a gap at the edges and the joining faces as described above and a fire ring is further engaged along the edge of the bore.
Such gaskets are referred to as soft gaskets. However, since asbestos is used in gaskets of this type, its use has become restricted, particularly in view of environmental problems.
As far as the inventor knows, synthetic fibers are used instead of asbestos in a gasoline engine. Further, for a diesel engine, a structure comprising a steel laminate including a steel sheet base plate formed with a minute bead, a stainless steel subplate stacked thereon and a steel wire core disposed to the bore peripheral edge is used. The subplate is folded back to cover the wire on the inside and extends along the upper surface of the edge of the bore holes. A baked rubber seal ring is disposed at the bolt holes.
The soft gaskets are not satisfactory for providing reduced thickness, endurance to cope with decreasing distance between bores, engine weight and size reduction, material recycling, heat resistance, high performance, cost reduction and freedom of design. Eventually the demand for metallic gaskets will increase in the automobile industry.
By the way, in a metallic gasket, a bead is formed along the periphery of holes, in particular, at the periphery of combustion chamber holes (hereinafter referred to as a bore hole or bore holes). When a metallic gasket is tightened between a cylinder head and a cylinder block, the bead is elastically deformed to function as a seal. Further, a stopper is formed, together with the bead, at the periphery of the bore hole in the metallic gasket. The stopper suppresses vibrations at the joining face area between the cylinder block and the cylinder head caused by the operation of the engine, so that it also functions as a sealing member to prevent fatigue failure on account of engine vibrations. In the metallic gasket, a complete sealing effect is intended by attaining a primary seal with the stopper and a secondary seal with the bead compensated with the durability by the stopper.
The stopper may be formed by folding back a subplate described above so as to cover the inner circumferential surface of the bore hole, or it may be formed by arc plasma spray on a gasket base plate or as a metal shim.
In any case, the stopper must have a thickness corresponding to a gap in the engine, that is, a gap between the joined surfaces of the cylinder block and the cylinder head.
Recently, with the performance improvement owing to the progress in engine technology, fuel economy and the size and weight reduction, the distance between bores has been narrowed. In addition, the combustion temperature and the explosive pressure have increased due to the adoption of super-chargers. Accordingly, with the prior art, the width of the stopper cannot be increased and it is difficult to impede the bead vibrations. Further, as the engine temperature becomes higher, the pressure exerted on the gasket surface increases because of the combustion expansion, and denting into the aluminum cylinder head is caused when a narrow shim stopper is used. Further, complete sealing has become difficult. A double seal with a stopper primary seal and a bead secondary seal cannot be obtained. In addition, since the shim stopper is not fixed to the base plate, it sometimes slides or becomes detached.
This invention has been accomplished in view of the foregoing situation and it is an object thereof to provide a metallic gasket with a shim, even if a distance between bores is narrow, which is also capable of adjusting the bead spring force.
As an example of the metallic gasket in the relevant prior art, the invention has accomplished an invention as disclosed in Japanese Patent Publication Hei 2 (1990)-58502 and, further is well aware of Japanese Patent Laid-Open Sho 62 (1987)-155376 and Japanese Patent Publications Hei 3 (1991)-20626 and Hei 4 (1993)-40540.